UPDATE: This looks like it is caused by running rpi-update to build #138
Problems did not exist before updating on 26/06/2012 to this build.
The kevent 4 message is constantly written to the syslog causing the SD to slow the system to a crawl.
Booting build #138 without an ethernet cable plugged in seems fine.
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Encountered this error today after running rpi-update with an MS natural keyboard and MS USB mouse plugged directly into the Pi.
Symptom was a non-working ethernet.
dmesg gives hundreds of lines of this error:
dmesg
eth0: kevent 4 may have been dropped
eth0: kevent 4 may have been dropped
eth0: kevent 4 may have been dropped
...
This thread here http://www.raspberrypi.org/phpBB3/viewtopic.php?f=28&t=7445&p=105118 seems to indicate it is power related.
I am currently using a 700mA USB charger from the ZTE blade so maybe I need to upgrade to an iPad charger as these can supply 2 Amps @ 5V.